The present invention relates to a decompression device of a four-stroke-cycle internal combustion engine to be mounted on an outboard motor for example, for reducing compression pressure of a cylinder in a compression stroke to facilitate starting of the engine.
Some four-stroke-cycle internal combustion engines having manual starting devices such as a recoil starter are provided with decompression devices for reducing compression pressure of a cylinder in a compression stroke to facilitate starting of the engine. A decompression device having a decompression pin provided in a cam shaft to project radially outward of the cam shaft has been known. According to this decompression device, when an engine is to be started, the decompression pin projecting radially outward of the cam shaft opens an exhaust valve by a small lift in the compression stroke to reduce compression pressure in a cylinder.
For example, in a decompression device of an internal combustion engine disclosed in Japanese Laid-Open Patent Publication No. 9-49408, a decompression shaft is rotated to project a decompression pin by centrifugal force. This internal combustion engine is a four-stroke-cycle internal combustion engine mounted on an outboard motor and has a recoil starter. The decompression device has a decompression shaft rotatably provided within a cam shaft, a decompression pin and a centrifugal clutch mechanism. The decompression shaft is formed with a cut at a part of the outer periphery near an exhaust cam and a pin hole is formed on a cam shaft at a position opposite to the cut. Into the pin hole is inserted a decompression pin slidably in a radial direction of the cam shaft. The centrifugal clutch mechanism is provided on an outside part of a cam shaft pulley which is connected with a crankshaft pulley by a timing belt wound round the pulleys, and has a pair of weights rotatably supported by support pins. The weight is rotated by centrifugal force acting on it to swing outward against force of a spring, and at that time, the decompression shaft engaging with the weight rotates within the cam shaft.
In the above-mentioned decompression device, when the engine is started, the engine rotational speed is low and the centrifugal force acting on the weight is small, so that the weight does not rotate against the spring force. In this state, since the decompression pin touches an outer peripheral part of the decompression shaft having no cut formed, a tip end part of the decompression pin projects by a predetermined length from a surface of the cam shaft corresponding to a base circle part of the cam, so that the exhaust valve is opened a little in the compression stroke to release compression pressure.
When the engine has been started, the engine rotational speed rises and the weight is rotated by centrifugal force, and at the same time, the decompression haft engaging with the weight rotates until the cut reaches a position opposite to the decompression pin. In that state, the decompression pin fits in the cut so as not to project from the surface of the cam shaft, therefore the exhaust valve is never opened in the compression stroke.
Japanese Laid-Open Patent Publication No. 8-21221 discloses a pressure reducing device (decompression device) of an internal combustion engine in which a working shaft (decompression shaft) is reciprocated by oil pressure to move a pin (decompression pin). The pressure reducing device has the working shaft provided within a cam shaft so as to reciprocate in the axial direction, the pin and a piston. The pin is fixed to the working shaft in a state that the pin projects from a cylindrical surface (base circle part) of an exhaust cam portion, and accommodated in a slot formed in the cam shaft. The piston receives oil pressure generated in an oil pump driven by the engine to touch an end of the working shaft and displace the shaft axially against force of a coil spring.
In this pressure reducing device, when the engine is started, the engine rotational speed is low and oil pressure acting on the piston is low, so that the working shaft does not displace against the spring force even if the piston touches the working shaft. In this state, since the pin is positioned at an end of the slot near the exhaust cam portion projecting from the cylindrical surface, the exhaust valve is opened a little by the pin in the compression stroke to reduce the compression pressure.
When the engine has been started, the engine rotational speed increases and the oil pressure generated in the oil pump increases, so that the piston displaces the working shaft axially against the spring force. In this state, the pin is positioned at an end of the slot remote from the exhaust cam portion, so that the pin engages with no rocker arm and the exhaust valve is not opened in the compression stroke.
Regarding the decompression device utilizing centrifugal force acting on the weight to move the decompression pin, though the decompression action can be canceled when the engine rotational speed exceeds a set value, in order to ensure the cancel it is necessary to obtain necessary working force by making the weigh heavy or lengthening the moment arm of the weight, therefore the weight is apt to be large-sized. Accordingly, in this decompression device, a larger space must be ensured around the cam shaft within a cylinder head compared with the decompression device utilizing oil pressure, so that the engine is caused to be larger and heavier.
Regarding the decompression device utilizing oil pressure generated in the oil pump driven by the engine to move the decompression pin, the decompression action is canceled when the oil pressure exceeds a set value and the device is small and light. However, in this decompression device, it is difficult to cancel the decompression action irrespective of oil temperature when engine rotational speed exceeds a set value